The German economy was already in poor condition, and it was the looting of Austrian gold and Czech armaments that gave it a temporary boost in what was effectively still peacetime. (The later looting of the Polish and French economies never made up for the costs of a full world war being in progress.)
Demographically German military manpower was at a height in 1940/41 that gave it an advantage over the allies and potentially the Russians, that would quickly evaporate within a few years. (Demographics was an important science between the wars, and many leaders – like Hitler and Stalin – made frequent references to it. The Russians in particular would start having more manpower available starting in 1942 … perhaps not a coincidence that Germany invaded in 1941?)
The Nazi air forces had a temporary superiority over the Allies in 1939 that was already being rapidly undercut as both the British and the French finally started mass production of newer aircraft. (By mid-1940 British aircraft production had overtaken the Germans, even without the French. If the war had not started in 1939, by 1941 the Luftwaffe would have been numerically quite inferior to the combined British and French air forces, even without the surprisingly effective new fighters being brought on line by the Dutch and others.)
German ground forces, while not really ready for war in September 1939 (half of their divisions were still pretty much immobile, and they had only 120,000 vehicles all up compared to 300,000 for the French army alone), were nonetheless in a peak of efficiency considering the Czechs and Poles had been knocked out, and the British and French were struggling to get new equipment into service. The Soviet short-term decision to ally with the Germans to carve up Eastern Europe (Stalin knew this was only a temporary delay to inevitable conflict), also allowed the Germans an easy victory and much greater freedom of action. Again, by 1941 British conscription and production, and French (and Belgian, and Dutch, etc.) upgrades and increases in fortifications, would have come a lot closer to making the German task next to impossible. (Even then it was the collapse of French morale after the loss of Finland — leading to the collapse of the French government – and Norway, that really defeated France, not vastly inferior divisions or equipment.)
A byproduct of an Allied ramp up might also have seen Belgium rejoin the allied camp in 1941, or at least make significant planning preparations to properly add its 22 divisions and strong border fortifications to allied defences if Germany attacked. (Rather than the hopeless mess that happened in 1940 when the allies rushed to rescue the temporary non-ally that had undermined the whole interwar defensive project …) Again, the Germans managed to find a sweet spot in 1939-40 that temporarily undermined long-standing interwar co-operation, and one that was not likely to last very long.
Similarly a delay of war would have allowed allied negotiations with the Balkan states to advance. The same guarantee that was given to Poland had been given to Yugoslavia, Rumania and Greece. (It is usually forgotten that Greece – attacked by Italy – and Yugoslavia – voluntarily – joined the British side at the worst possible moment in 1941. (Only to be crushed by the Germans … but with the interesting by-product of effectively undermining Germany’s chances of defeating the Soviets and occupying Moscow in the same year …)
Nigel Davies, “If the War hadn’t started until December 1941, would it?”, rethinking history, 2021-05-01.
